The invention relates to a hydraulic brake force amplifier. A brake force amplifier of this type is known from German Offenlegungsschrift 27 50 491 which has a corresponding U.S. Pat. No. 4,197,710 which is incorporated herein by reference.
Fundamental to this known design is the problem of travel path simulation, which is intended to be limited, when the supply of pressure is intact, in order to keep pedal travel at a minimum.
As is well known, hydraulic brake force amplification systems can be classified as either auxiliary force or external force system. In this connection, the terms "open" and "closed" brake circuits are often used. The first, "open" brake circuits are associated with the external force systems. Intermediate forms are also known in the state of the art, where one circuit is open and one circuit is closed. Such systems have the disadvantage that if the pressure supply fails one brake circuit also fails at the same time.
In auxiliary force amplification systems, a distinction is made for systems where the amplifier portion and the main brake cylinder are disposed together in a tandem arrangement. These systems function without travel path simulators. The pressure volume characteristic of the cooperating brake circuits determines the pedal travel at a particular time. The pedal characteristic is given according to the amount of brake force amplification. These systems are disadvantageous in the event of brake circuit failure, because in that case the pedal travel path is lengthened.
This disadvantage is avoided in systems which utilize a travel path simulator. For human engineering reasons, a relatively short pedal travel path is desired for fully applying brake pressure. If the pressure supply fails, on the other hand, the entire pedal travel path should be utilized for generating brake pressure via the main brake cylinder piston. For these reasons, the state of the art, according to the cited German Offenlegungsschrift 27 50 491, makes use of an arresting piston member, which limits the pedal travel path, or the path simulator's travel, after full brake pressure has been applied. This arresting piston member is not permitted to function if the pressure supply fails. The switchover of the arresting piston member function is accomplished by means of switching valves. These interrelated factors mean that the basic expenditure for a hydraulic amplification system of this kind comprises a brake valve with a travel path simulator, two main brake cylinder pistons and one arresting piston member with switchover valves. The basic expenditure is thus relatively high.